In recent years, soft silicone hydrogel contact lenses become more and more popular because of their high oxygen permeability and comfort. “Soft” contact lenses can conform closely to the shape of the eye, so oxygen cannot easily circumvent the lens. Soft contact lenses must allow oxygen from the surrounding air (i.e., oxygen) to reach the cornea because the cornea does not receive oxygen from the blood supply like other tissue. If sufficient oxygen does not reach the cornea, corneal swelling occurs. Extended periods of oxygen deprivation cause the undesirable growth of blood vessels in the cornea. By having high oxygen permeability, a silicone hydrogel contact lens allows sufficient oxygen permeate through the lens to the cornea and to have minimal adverse effects on corneal health.
Typically, silicone hydrogel contact lenses are produced according to a cast molding technique involving use of disposable or reusable molds and a silicone hydrogel lens formulation (i.e., a mixture of vinylic monomers and/or vinylic macromers). A silicone hydrogel lens formulation often comprises a bulky siloxane-containing vinylic monomer, such as, for example, a vinylic monomer having a tris(trialkylsilyloxy)silylalkyl group (e.g., tris(trimethylsilyloxy)-silylpropyl acrylate, tris(trimethylsilyloxy)-silylpropyl methacrylate, tris(trimethylsilyloxy)-silylpropyl acrylamide, tris(trimethylsilyloxy)-silylpropyl methacrylamide, tris-(trimethylsiloxysilyl) propylvinyl carbamate, etc.). It is reported that such a bulky siloxane-containing vinylic monomer is critical to the elimination of optical defects derived from handling during manufacturing, especially when curing the monomer mixture in a mold within a relatively short time (e.g., less than about 300 seconds) with a UV light. When such a bulky siloxane-containing vinylic monomer is eliminated from a monomer mixture for making silicone hydrogel contact lenses, resultant lenses may develop permanent deformations (optics defects) due to handling, whereas when such a bulky siloxane-containing vinylic monomer is present, resultant lenses exhibit a ‘healing’ effect that eliminated the optical defects (i.e., the folding marks become transient and can disappear after a short time period, e.g., about 15 minutes or less). However, silicone hydrogel lenses produced from a lens formulation known in the art may not have a desired thermal stability.
Therefore, there is still a need for silicone hydrogel contact lenses which have a relatively long thermal stability and can be produced by curing a lens formulation in molds within a relative short period of time (e.g., less than about 100 seconds).